There continues to be speculation regarding whether lichens could survive on Mars. Lichens have been described as %91extremophiles%92 and could potentially withstand some aspects of the hostile environment of the planet especially if they are %91endolithic%92 and live within the rocks as in some deserts and dry valleys in Antarctica. Lichens, however, are composite organisms and their existence assumes the successful establishment of a variety of microorganisms on Mars including eukaryotic green algae, blue-green algae (cyanobacteria), and various types of fungi. To date, although %91lichen-like%92 and %91fungal-like%92 structures have been described from the surface of Mars, there is no conclusive evidence of the presence of lichens or their symbionts on the surface. If lichens are present on Mars, they would be subjected to a considerably more hostile environment than the most extreme on Earth, which is regarded as at the limit of tolerance of present day lichens. The probable lack of sufficient liquid water over most of the surface and the problem of obtaining sufficient nitrogen and other nutrients are particular problems that Martian lichens would have to overcome. This review introduces various aspects of the lichen symbiosis and discusses the adaptation of lichens to extreme conditions on Earth with special reference to those living in very dry and/or cold environments such as deserts and the polar regions. Given their considerable adaptability, the pros and cons of whether lichens and their symbionts could survive on Mars, and the theoretical attributes of a %91putative%92 Martian lichen, are discussed. LICHEN, PHYSIOLOGY, ADAPTATION, ARCTIC AND ALPINE ENVIRONMENTS, ANTARCTICA, DESERTS, EXTREMOPHILES, MARS